WEDNESDAY, July 18 (HealthDay News) -- Researchers have uncovered the first evidence that the adult human lung is capable of growing back -- at least in part -- after being surgically removed.

In an observational study, researchers used MRIs with hyperpolarized helium-3 gas to show that existing alveoli -- the tiny, air-exchange units of the lung -- actually increased in number after a 33-year-old woman had her entire right lung removed due to
cancer.

The study showed a 64 percent increase in the number of alveoli in the woman's lung 15 years after surgery. "The research clearly shows that some form of lung growth can occur in the adult human," said study author James Butler, an associate professor of medicine in the department of medicine at Harvard Medical School in Boston.

The new alveoli were all shaped similarly. "It's striking, the degree of homogeneity of the new alveoli, as if the lung was responding to something," Butler added. The cause of the new growth could be stretching of the tissue, perhaps by exercise, he suggested. "Could other bio-molecular growth be triggered by stretch? It's a wide-open question now."

About a year and a half after surgery, the woman began a daily exercise program including walking, cycling and yoga. Previous studies in adult dogs have suggested that lung growth after pneumonectomy (removal of the lung) in dogs was possible, typically after periods of lung stress or strain.

Over a period of 15 years, data measuring lung size and capacity were collected, using common respiratory tests (called FEV and FVC) measuring how much air can be taken in and blown out with deep breaths. In the early months after surgery, the lung responded as researchers would expect. The total lung volume increased and the lung density fell below normal. But, the lung tissue volume gradually started to increase and the density returned to a level normally seen when a deep breath is taken, suggesting the growth of new tissue.

The ability of the lung to regenerate, potentially triggered by exercise, makes sense, said Dr. Norman Edelman, a professor of medicine at Stony Brook University and chief medical officer of the American Lung Association. "When the lung develops in utero [when the fetus is developing], the pulling force of the diaphragm is an important stimulation for the lung to grow," he said. "But, of course, the practical application of the research is a long way off."

Butler said the next step is to do a study involving more people over time. "If we can discover the underlying bio-molecular mechanisms, they would suggest potential therapeutic options," he explained.